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SYNTHETIC COMMUNICATIONSV
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Scheme 2. Reaction of benzaldehyde, acrylonitrile, carbon disulfide and benzyl amine.
Results and discussion
We started our study using benzaldehyde 1a, acrylonitrile 2a, carbon disulfide and ben-
zyl amine as a model reaction (Scheme 2). For optimization of the reaction conditions,
the reported protocols26,27 by Saika and Sakakura were applied for BH and acetylation
steps, respectively. We observed that mixing of benzaldehyde (0.5 mmol, 1 equiv.),
acrylonitrile (3 equiv.), and DABCO (1 equiv.) under solvent free conditions for 12 h
afforded the corresponding BH adduct in quantitative yield. Then acetic anhydride (1.2
equiv.) as the acetylation agent and DMAP (0.2 equiv.) as catalyst were added to the
reaction vessel and the resulting mixture was stirred for 10 minutes at the same condi-
tions to afford the acetylated BH product. Finally, nucleophilic displacement of the
resulting BH acetate with dithiocarbamic acid was carried out by adding carbon disul-
fide (1.5 equiv.) and benzyl amine (1 equiv.). Under these conditions, the corresponding
(E)-2-cyano-3-phenylallyl benzylcarbamodithioate 3a was obtained in 38% yield. In
order to improve the reaction yield, the last step was screened in various solvents such
as EtOH, THF, DMF, CH2Cl2, and water. The yield was improved to 62% in THF and
DMF. Furthermore, reducing the reaction temperature to 0 ꢀC during the addition of
amine and CS2, and further stirring for 1 h at room temperature improved the yield of
3a to 76%. In summary, performing the BH reaction and acetylation step under solv-
ent-free conditions at room temperature, followed by the addition of CS2 and an amine
in THF at 0 ꢀC and further stirring at rt for 1 h was considered as optimal reaction con-
ditions for further derivatization.
Under optimized reaction conditions, the generality of this reaction was investigated
using various aromatic aldehydes, activated alkenes and primary and secondary amines
and the results are summarized in Table 1. Various aromatic aldehydes such as benzal-
dehyde, 4-chlorobenzaldehyde, 3-nitrobenzaldehyde, and thiophene 3-carbaldehyde were
applied successfully in this protocol. In addition, various primary and secondary amines
such as benzyl amine, isobutyl amine, propylamine, butylamine, dimethylamine, diethyl-
amine, and pyrrolidine are suitable substrates in this protocol. While in the reaction of
aldehydes with acrylonitrile, the E-stereoisomer was obtained with excellent stereoselect-
ivity, the co rresponding Z-stereoisomer was achieved with methyl acrylate. The stereo-
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chemistry of the products was determined by comparing the H NMR value of alkene
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proton with literature values.[10,11,28] The structure of products was deduced by H
NMR, 13CNMR, FT-IR and CHN analyses.
Reaction mechanism is proposed in Scheme 3. Baylis–Hillman reaction of acrylo-
nitrile with benzaldehyde provides the corresponding BH intermediate, which can be
easily acetylated with acetic anhydride in the presence of DMAP to afford BH-acetate.
Finally, nucleophilic displacement (SN20) of the BH-acetate with in situ prepared dithio-
carbamic acid afforded the product with removal of the acetate group.